1. Field of the Invention
The present invention relates to a semiconductor device and a protection method, and more in particular to a power control semiconductor device and a protection method capable of protecting a switching element from a high voltage applied in off state or in a state leading to an off state.
2. Description of the Related Art
In recent years, an IGBT (Insulated Gate Bipolar Transistor) has been closely watched as a power control semiconductor device. The IGBT, which is a bipolar device having a MOS structure, has a high-speed switching characteristic of a power MOSFET and high withstand voltage, and high conduction characteristics of a bipolar transistor.
The IGBT of this type is used as a main switching device connected with the drain electrode D thereof as a positive potential side and the source electrode S thereof as a negative potential side in a series circuit of a DC power supply 301 of E1 in voltage and a motor coil 302, as shown in FIG. 1. The potential of the gate electrode G of the IGBT is controlled by a gate control circuit 303 thereby to turn the series circuit on and off.
An on-off control timing chart is shown in FIG. 2, for example. Character V designates a drain-source voltage (hereinafter referred to as the main voltage), and I designates the drain-source current or the current in the series circuit. Vg designates a gate voltage.
Now, assume that Vg is 0 and IGBT is off (time t=T1). In other words, assume that I is 0, and V is substantially equal to a power source voltage value. Next, it is assumed that a predetermined voltage is applied to Vg and the IGBT is turned on (time t=T2). When the IGBT is turned on, V is rapidly reduced while I gradually increases. In this case, a coil 302 stores energy of (1/2)LI.sup.2 when the self-inductance is assumed to be L.
Next, assume that Vg is 0 and that IGBT is turned to off state (time t=T3). When IGBT is turned off, V rapidly rises to a value higher than the power source voltage value E1 and then settles at the power source voltage value E1. The rapidly rising portion of V occurs as the IGBT receives the coil energy when the IGBT changes from on to off state. Like other semiconductor devices, the IGBT is highly liable to break down upon application thereto of a voltage higher than the voltage allowed for the operation (hereinafter referred to as the overvoltage). It is therefore necessary to protect the IGBT by suppressing the overvoltage of the rapidly-rising V portion below a certain value.
The IGBT is liable to reach to an overvoltage state when it is off or transferring to an off state. In other words, the overvoltage condition of the IGBT occurs when the gate voltage Vg is zero or assumes a low negative value, or the gate voltage Vg is reduced. If the IGBT is to be protected, therefore, the gate voltage Vg is required to be increased so that the IGBT is turned on incompletely and thereby the IGBT resistance is reduced to prevent the main voltage from increasing beyond an allowable value.
The IGBT has its own function of preventing a voltage of a predetermined value or more from being applied thereto. This is called the self-clamping function, and the value of the voltage thereof is called the self-clamp voltage. The self-clamp function, however, is difficult to control since it is a current conduction attributable to the avalanche phenomenon due to a high electric field. The IGBT thus is liable to break down due to local current concentration within itself.
In order to protect the IGBT from the overvoltage at the time of turning off, a zener diode ZD is conventionally connected between the drain and gate as shown in FIG. 3. Also, the gate is connected to a gate drive circuit 303 through a resistor Rg. Now, this protection circuit will be explained in detail with reference to FIG. 4.
FIG. 4 is a sectional view schematically showing an IGBT of this type and a peripheral configuration thereof. This IGBT includes a drain electrode 312 formed on a p-type drain layer 311. An n-type base layer 313 is also formed on the surface of the p-type drain layer 311 opposite to the drain electrode 312. A plurality of p-type base layers 314 are selectively formed by diffusion in the surface of the n-type base layer 313. A plurality of n-type source layers 315 are selectively formed in the surface of each p-type base layer 314.
A gate electrode 317 is formed through a gate insulating film 316 on each p-type base layer 314 formed between each n-type source layer 315 and the n-type base layer 313. Also, a source electrode 318 is formed on each p-type base layer 314 and the n-type source layers 315.
Each gate electrode 317 is connected to a gate drive circuit 303 through a gate resistor Rg.
Further, a zener diode ZD is interposed between the gate electrodes 317 and the drain electrode 312 in order to protect the IGBT from the overvoltage described above.
If this IGBT is to be turned on, a voltage positive with respect to the source electrodes 318 is applied to the gate electrodes 317 with a voltage (main voltage) positive with respect to the source electrodes 318 being applied to the drain electrode 312. As a result, a n-type channel is formed in the surface of the p-type base layer 314 held between the n-type base layer 313 and the n-type source layer 315, so that an electron current flows into the n-type base layer 313. On the other hand, a hole current flows into the n-type base layer 313 from the p-type drain layer 311. Thus a conduction modulation occurs in the n-type base layer thereby to turn on the IGBT.
If the IGBT is to be turned off, on the other hand, a zero or a negative voltage with respect to the source electrode 318 is applied to the gate electrode 317. As a result, the n-type channel disappears and electron injection into the n-type base layer 313 ceases, and the IGBT soon turns off. The main voltage is applied even in this state.
The IGBT is liable to suffer an overvoltage when in an off state or transferring to the off state.
In the configuration of FIG. 4, a zener diode ZD is provided for preventing the overvoltage condition. This zener diode ZD has a zener voltage (peak inverse withstand voltage) lower than the self-clamp voltage of the IGBT. A current flows in the zener diode ZD when an inverse voltage higher than the zener voltage is applied thereto. As a result, when the main voltage higher than the zener voltage is applied between drain and source of the IGBT, a current flows through the zener diode ZD to the gate drive circuit 303 from the drain of the IGBT. Consequently, the voltage drop due to the gate resistor Rg increases the gate voltage Vg and turns on the IGBT incompletely thereby to protect the IGBT from an overvoltage.
The use of a zener diode thus can prevent the breakdown of the conventional IGBT but poses the problem described below.
First, when the drain-source main voltage rapidly rises to a value higher than the power source voltage value due to the inductance, the zener diode ZD protects the IGBT from an overvoltage. In the process, however, the negative resistance of the zener diode ZD generates a large noise. This noise in turns causes a gate operating error and the breakdown of the IGBT.
Also, the zener voltage across the zener diode ZD is subject to considerable variations. Even when the same type of zener diode ZD is used in the same circuit, therefore, the same voltage is not necessarily generated for protection. Due to the variations in zener voltage, the protection voltage (clamp voltage) is difficult to design. Further, the clamp voltage cannot be changed once the zener diode ZD is built in the circuit. It is therefore difficult to change the clamp voltage in accordance with the operating conditions, temperature or circuit conditions of the IGBT.
Also, since the IGBT has a high operating voltage, the clamp voltage also assumes a high value. The zener voltage of the zener diode ZD, however, assumes a value considerably lower than the clamp voltage of the IGBT. Actually, therefore, the zener diode ZD in FIG. 4 is configured of a series circuit of a plurality of zener diodes, leading to the problem that a large area is required.
Furthermore, in the case where the rate dv/dt at which the main voltage rises is large, the IGBT may be broken. This is called the dv/dt breakdown, which is difficult to prevent for the IGBT using a zener diode ZD.